Introducing new chicks to the old ones

We have two nine week olds and just got four more three week olds. The breeder said that it would be fine to put them together. But the older ones are pecking at the little guys. Does anyone have any suggestions on how to get them to become friends?

Back Yard Poultry has a great article on just that this month. I won't go into all the details bit it states all birds have a pecking order and they must fall in line. They suggest keeping the birds in the coop in a separate caged so they can react without being able to get to one another. The new birds need to know that coop is their home too. After a few days you can introduce the new birds together and it is best if you can do it in a large area. The birds will still show aggression to the new ones but it never seems to last long. You need to keep an eye on them but you must look away from time to time. I will admit it is tough to watch but things will work out. It is best to wait till all are about the same size. Good luck. John

I also have chickens of different ages. When the new chickens were old enough to go outside I built a small run attached to the older chickens run. This way they are in close proximity and have some interaction daily. I won't put the new babies in with the older chicks until they are the same size.
I am hoping this makes the introduction less stressfull.